For beginning or prospective racers, the Valley Spokesmen have a pace line ride which starts from Shannon Park on San Ramon Blvd at 6pm sharp every Tuesday & Thursday. See their website, then select ride calendar to see specifics.
Usually 12-to-25 riders show up of varying abilities, for about 1 1/2 hours, averaging 18-19 mph. The group is not fast enough for racers.
If you are too fast for this VS group, then try the House Of Pain Lite ride which starts from Peet's in downtime Danville at 8:45 am on Saturday mornings. They do the same 50 mile loop every Saturday, with 60-to-80 Cat 4's and 3's racers. The ride is temperate out Sycamore/Camino Tassajara, then the tempo picks up dramatically out Highland to Livermore.